Ground fault protection for personnel and electrical equipment is an ever increasing concern in both home and industry. For residential circuit applications, ground fault circuit interrupting (GFCI) devices are now widely available in compact circuit breaker and receptacle configurations for convenient installation in existing service entry equipment and wall receptacle boxes. These GFCI devices are primarily intended to protect people from the hazards of electrical shock caused by leakage current emanating from ground faults, however they do afford a measure of equipment protection in terms of acting to halt ground fault current which can cause fires.
For high current applications found in industry, ground fault protection is available in basically two configurations. With the event of so-called "static trip" circuit breakers, it has become economically possible to combine overcurrent and ground fault signal processing circuitry in a compact electronic trip unit package which can be integrated with the circuit breaker to achieve comprehensive circuit protection. Alternatively, traditional industrial circuit breakers having thermal-magnetic and dual-magnetic trip units for overcurrent protection can be utilized with so-called "ground fault relays" in providing ground fault protection as well. These relays respond to a ground fault signal developed by a suitable sensor, such as a zero sequence transformer coupled with the load carrying conductors, by energizing (or de-energizing) a solenoid which, in turn shunts trips the breaker to initiate circuit interruption.
U.S. Pat. No. 4,044,395, issued Aug. 23, 1977 assigned to the assignee of the instant application, discloses a system for protecting relatively large power distribution circuits against ground faults by utilizing as its principle operating component a conventional GFCI device normally used in low power, residential circuit applications. As disclosed and claimed therein, the GFCI device is installed in a control circuit for an undervoltage release solenoid adapted to a conventional circuit breaker protecting the distribution circuit or in the control circuit for the holding coils of a contractor operating in the distribution circuit. A ground fault sensor in the form of a zero sequence transformer coupled with the load current carrying conductors of the distribution circuit develops a current signal in its secondary winding in response to a ground fault on the distribution circuit. This current signal is injected into one side of the control circuit to create a current imbalance of the nature to which the GFCI device is responsive. The GFCI device trips to interrupt the control circuit, and the undervoltage release solenoid drops out to trip the circuit breaker or the holding coil becomes de-energized to open the contactor. In either case, the distribution circuit is interrupted to clear the ground fault.
It is an object of the present invention to provide an inexpensive ground fault protection system suitable for high current, industrial circuit applications.
An additional object of the present invention is to provide a ground fault protection system of the above character which is adaptable to different types of industrial circuit current interrupters.
A further object of the present invention is to provide a ground fault protection system of the above character, wherein the ground fault circuit interrupting (GFCI) device is equipped with both normally open and normally closed switch contacts individually capable of functioning to control the operation of current interrupting devices in interrupting an industrial circuit pursuant to clearing a ground fault thereon.
Other objects of the invention will in part be obvious and in part appear hereinafter.